Keeping in step with the growth of the computing and communications industries has been the ever increasing need for affordable and convenient portable electronic devices, such as mobile telephones, computers, fax machines, games, lighting, audio equipment, medical equipment and the like. In answering this need, one facet of the industry has focused much attention on developing smaller, lighter, less expensive, and energy efficient electronic devices. However, power demands have dramatically increased as new features are introduced in portable devices. For instance, typical portable computers include color displays, internal modems and large banks of memory. In parallel with the development of portable devices, another facet of the industry has focused on developing smaller, lighter, less expensive, and energy efficient batteries. As these two facets of industry develop, the increasing longevity of the operational use of various portable electronic devices while optimizing the size and weight remains a major goal. The present invention is directed toward taking the next step in this technological revolution.
Today, users of portable electronic devices commonly purchase several batteries that are compatible with the device. Users carry these batteries with them and change the batteries out as they become discharged. Although this technique allows users to be mobile for longer periods of time, there are several disadvantages associated with this practice. The cost of the additional batteries can be quite significant. In addition, the process of maintaining a supply of charged batteries and carrying them around can be very burdensome. The weight of the additional batteries is a problem for someone who needs a high degree of mobility. Users must change batteries at inopportune moments, such as the middle of a conversation on a portable telephone, downloading information with a portable computer, or while operating a video camera in the middle of their son's turn at bat. Thus, there is a need for a portable energy source that is inexpensive, light, small, efficient, and minimizes the interruptions to the operation of portable electronic devices by increasing the longevity of the operational use.
One possible technique to limit the interruption of operation is by placing multiple batteries in parallel. This technique extends the useful operating time of the electronic device, however, the length of time ("RUNTIME") that the parallel batteries can source the electronic device is a function of the characteristics of the batteries. RUNTIME is defined as the period of time that multiple batteries can continually drive a load without having to be recharged. As an example, assume that two batteries are connected in parallel to drive a load. If the wattage required by the load is WLOAD, then the upper threshold of the RUNTIME can be determined as: EQU RUNTIME=(V.sub.B1 C.sub.B1 +V.sub.B2 C.sub.B2)/WLOAD,
where V.sub.B1 is the voltage and C.sub.B1 is capacity in amp-hours of one battery and V.sub.B2 is the voltage and C.sub.B2 is the capacity in amp-hours of the other battery. By adding additional batteries in parallel, the RUNTIME can be extended even further: EQU RUNTIME=(V.sub.B1 C.sub.B1 +V.sub.B2 C.sub.B2 . . . +V.sub.Bn C.sub.Bn)/WLOAD.
Simply connecting multiple batteries in parallel has several limitations. To actually achieve the upper threshold RUNTIME, each of the batteries must be able to individually drive the load. For instance, if two batteries are connected in parallel but only one of the batteries, a dominant battery, is able to individually drive the load, the RUNTIME is limited by the characteristics of the dominant battery. Thus, there is a need for a system and a method for allowing multiple batteries with varying capacities and power capabilities to drive a common load while distributing the demand placed on each of the batteries in accordance with its capacity and power capability. This technique will allow a battery with a higher capacity and low power and a battery with a lower capacity and high power to commonly drive a load and achieve the upper threshold RUNTIME. Further, the combination of two different electrochemical systems may provide a hybrid with optimal performance dimensions generally not found in the same electrochemical package.